I had the same instinctive reaction as you; but I've now changed my mind a lot and I don't think they were too brusque at all.
To be frank, I think they were fully justified to find - and express - their anger at this. It's blatantly obvious to everybody that you aren't helping by anonymously dumping fresh food on their doorstep; in fact, they must realise that they're just creating more pointless work for people who are already busy doing very good work for no financial reward.
Nobody could genuinely think that they would (eventually) find the fresh food and think, with a big happy grin, "Ooh, we've no idea who left that there, or when, and how long it's been out of the fridge, and if there's anything wrong with it or it's been tampered with, weather-damaged or soiled by an animal... BUT nevertheless, that's exactly the kind of donation that we can use!!!"
Our local charity shop leave a huge sign outside their front door when the shop is closed telling people unequivocally NOT to leave donations on their step when they're closed. There is always stuff that's been fly-tipped right in front of the sign, every single time I pass by. People know very well that they are actively causing problems - but they exploit the fact that they can both make their rubbish somebody else's problem AND reframe the facts on SM to boast about how generous they are when it comes to 'giving' to good causes in the community.
It's an aggressive act, albeit dressed up as a caring one - why would they feel the need to tread carefully and make sure they don't risk upsetting somebody who is deliberately sabotaging their good work?